This invention relates to an automatic exposure control apparatus for obtaining ideal exposure using a video camera, by way of example.
A video camera or the like conventionally is provided with an automatic exposure control apparatus for maintaining exposure in an ideal state at all times.
In such an automatic exposure control apparatus, the diaphragm of an optical system and the level of a video signal usually are controlled in such a manner that the output video signal level remains constant. Conventionally, a circuit which performs averaging with regard to the entirety of one frame is used as a photometer circuit for controlling the diaphragm of the optical system and the video signal level. However, when the difference in luminance between the subject and the background is great, the subject appears too dark or black if the scene is backlighted and too bright or devoid of color if the scene is front-lighted. The result is an unnatural picture in either case.
In order to prevent the foregoing problem, a system has been proposed in which photometry is directed preferentially to a portion of the picture, e.g., the central portion (usually within a rectangular area) where the subject has a high probability of being located. Since the position of the photometric area is fixed within the picture, this method of photometry is referred to as fixed-area preferential photometry.
FIG. 1 is a block diagram showing the construction of the conventional automatic exposure control apparatus, which employs fixed-area preferential photometry, in an ordinary video camera. The apparatus includes an image pick-up optical system 10, a diaphragm 12 for regulating the quantity of incident light, an image sensor 14, a buffer amplifier 16, an automatic gain control (AGC) circuit 18, a photometer circuit 20 for controlling the diaphragm 12, a diaphragm drive circuit 22 for driving the diaphragm 12 in accordance with the output of the photometer circuit 20, a photometer circuit 24 for controlling the gain of the AGC circuit 18, a photometric area generating circuit 26 for outputting an area signal which designates the photometric areas of the photometer circuits 20, 24, a video-signal output terminal 28, and an input terminal 30 for a composite synchronizing signal.
The photometer circuit 20 controls the diaphragm 12 via the diaphragm drive circuit 22 in such a manner that the output level of the image sensor 14 is rendered constant. The photometer circuit 24 controls the gain of the AGC circuit 18 in such a manner that the output signal level of the AGC circuit 18 is rendered constant.
FIG. 2 illustrates one example of the construction of the photometer circuit 20. The photometer circuit 24 has a construction identical with that shown in FIG. 2. As shown in FIG. 2, the photometer circuit 20 includes an analog switch 31, low-pass filters (LPF) 32, 34, resistors 36, 38, and a buffer amplifier 40.
The output video signal from the buffer amplifier 16 is applied to the LPF 32 directly and to the LPF 34 via the analog switch 31. The analog switch 31 is opened and closed under the control of the area signal outputted by the photometric area generating circuit 26. The portion of the switch 31 corresponding to the photometric area of the video signal is placed in the closed state so that this portion of the video signal is delivered to the LPF 34.
More specifically, the LPF 32 outputs an average signal indicative of the average of the entire frame, and the LPF outputs an average signal indicative of the portion of the frame within the photometric area. These average signals are added together upon being weighted by the resistors 36, 38, respectively, and the sum is outputted by the buffer amplifier 40.
If the settings of the resistors 36, 38 are controlled to enlarge the weighting of the output from the LPF 32, photometry will approach that for the average of the entire frame. Conversely, if the resistance settings are controlled to enlarge the weighting of the output from the LPF 34, photometry will approach that which stresses the picture within the photometric area.
With fixed-area photometry according to the example of the prior art described above, certain problems are encountered.
Specifically, when photography is performed outdoors, a backlighted state in which the sky enters at the top of the picture often occurs. In such case, when preferential photometry is applied to the subject and it is attempted to avoid excessive darkening of the subject, it is desired that the preferential photometric area fall within the subject, as illustrated in FIG. 3A. However, if the subject is a person wearing clothing (such as a black suit or coat) having a low level of luminance, as shown in FIG. 3B, the dark clothing enters the photometric area and therefore the average luminance declines. Since control of the diaphragm is performed in accordance with the average luminance, the face of the person serving as the subject becomes to bright and colorless.
Even if the clothing worn is not dark, as shown in FIG. 3C, average luminance will decline and the person's face will appear too bright if the only the face is bright (as when the person is in a spotlight).